Change -Wbind-to-temporary-copy from an ExtWarn to an Extension.

The reasoning is that this construct is accepted by all compilers and valid in
C++11, so it doesn't seem like a useful warning to have enabled by default.
Building with -pedantic, -Wbind-to-temporary-copy, or -Wc++98-compat still
shows the warning.

The motivation is that I built re2, and this was the only warning that was
emitted during the build. Both changing re2 to fix the warning and detecting
clang and suppressing the warning in re2's build seem inferior than just giving
the compiler a good default for this warning.

Also move the cxx98compat version of this warning to CXX98CompatPedantic, and
update tests accordingly.